For internal combustion engines, such as diesel engines, nitrogen oxide (NOx) compounds may be emitted in the exhaust. To reduce NOx emissions, a selective catalytic reduction (SCR) process may be implemented to convert the NOx compounds into more neutral compounds—such as diatomic nitrogen, water, or carbon dioxide—with the aid of a catalyst and a liquid reductant. The catalyst may be included in a catalyst chamber of an exhaust system, such as that of a vehicle or power generation unit. A liquid reductant—such as anhydrous ammonia, aqueous ammonia, diesel exhaust fluid (DEF), or aqueous urea—is typically introduced into the exhaust gas flow prior to the catalyst chamber.
To introduce the liquid reductant into the exhaust gas flow for the SCR process, an SCR system may dose or otherwise introduce the liquid reductant through a dosing module that vaporizes or sprays the liquid reductant into an exhaust pipe of the exhaust system up-stream of the catalyst chamber. An amount of the liquid reductant that is sprayed into the exhaust pipe is metered by controlling a valve or a pump which provides the liquid reductant to the valve.
In some applications, the valve is provided the liquid reductant from the pump at a fixed or variable pressure. In these applications, the amount of liquid reductant that is sprayed into the exhaust pipe can be varied by changing a position of the valve. In other applications, a fixed displacement pump is controlled, by varying a number of pump strokes, to directly meter the amount of liquid reductant that is sprayed into the exhaust pipe without the use of a valve. However, none of these approaches provides multiple flow paths, each having an individually controllable valve and an orifice that provides the reductant to a common outlet at a target flow rate such that a customized flow rate can be achieved by controlling the valves. Therefore, none of these approaches offers control of a turndown ratio (e.g., ratio of maximum flow rate to minimum flow rate, etc.) through the addition and subtraction of flow paths, orifices, or valves.